A braking system typically includes a master cylinder which is fluidly coupled to downstream braking circuits. During an initial period of actuation, the master cylinder generates fluid pressure in downstream braking circuits and displaces fluid in order to place friction members of the braking system, e.g., brake pads, against complementary surfaces, e.g., a rotor or a drum. In certain circumstances, brake pads may be displaced away from the rotor, thereby generating a gap between the brake pads and the rotor. Thus, when actuation is first initiated the brake pads are not in physical contact with the rotor of the wheel.
This lack of physical contact between a brake pad and a rotor precludes any physical braking until the brake pads are repositioned into contact with the wheel rotors. Moreover, since there is no significant resistance in the system, there is only minimal pressure buildup in the downstream braking circuits. Consequently, in addition to the lack of braking, an operator of the vehicle may receive a different pedal feedback when braking is first initiated as compared to the pedal feedback the operator receives once the brake pads are in contact with the rotor. This difference in the pedal feedback can be unsettling to the operator.
One way to shorten the lack of braking and reduce the unsettling difference in the pedal feedback when the actuation is first initiated is to displace a larger quantity of fluid within the braking system in order to quickly take up the gap, described above. Such systems are typically referred to as a “fast-fill” braking systems. In order to transfer the larger quantity of fluid, the braking system may include an actuating piston in the master cylinder with a larger diameter as compared to an actuating piston in a braking system which is not designed to provide the desired fast-fill function. A larger diameter piston moves a larger volume of fluid, thereby quickly filling the downstream braking circuits.
A larger piston, however, requires a larger force to move. While during the initial period of actuation the force required to move the larger piston is relatively low, after the initial period of actuation a larger force is required to move the piston than is needed in a system with nominally sized piston. This additional force necessitates a larger boost system.
Therefore, it is highly desirable to provide a master cylinder construction which can minimize the lack of braking and reduce the unsettling difference in the pedal feedback when the actuation is first initiated by rapidly increasing pressure in the downstream braking circuits, and without the need to use a larger boost system.